BEST fullrange speaker drivers in existence, 4"-10" diameter range, and...Go!

effectively i must distinguish for myself whether or not, all other things being equal, that horns either are or are not decisively better (aka more "musical") than even the best speaker cone driver

I would check out the Pure Audio Project Horn 1 compression driver. I think its about 40 Hz to 20K. They also have the Voxativ versions (there are three, but VERY pricey).

your idea of using large fullranges in multiples is highly flawed rethink read study up on design instead of making such mistakes you could use a few 4inch but even thats not the best way loudspeaker design is not easy its not just slapping parts in a box

I use single 4" Merrill drivers in my Zigmahornets.  I would never think of doubling or quadrupling these.  The imaging is amazing and you won't get that with multiple drivers, just more sound.  Most single drivers tend to be very efficient with power.
The lack of a crossover is very clearly a benefit with this design.
Another option would be a coax driver to maintain the absolute best imaging (in my opion).  Seas makes a wonderful 6" coax that can be biamped/biwired or single wired with a jumper.  The crossover is absolutely minimal in their design.  Not a cheap option but amazing sound.

I would heed the advice of Johnk,  his comment makes much sense and reflects wisdom. Cube Audio (Poland) produces very highly regarded 8" and 10" full range drivers. Not cheap but are considered world class quality. 
Charles